Miniaturized devices having ultrasensitive ion detection capability are useful as ion detectors in diverse applications, including detection of radioactive material or other sources of radiation, electron/ion beam calibration, monitoring of pressure or vacuum, and detection of energetic particles from outer space. Carbon nanotubes (CNT) have been proposed as an ion sensing element in such devices (Modi et al., 2003).
Carbon nanotubes have attracted significant attention for use as sensors of gas molecules because of their extremely high surface-to-volume ratio and their hollow structure, which is advantageous for adsorption of gas molecules (Mattmann et al., 2010; and reviewed in Wang and Yeow, 2009). Generally, upon exposure of CNT to gas molecules, charge transfer occurs between the gas molecules and the CNT. As a result, the gas molecules act either as electron donors or electron acceptors, thereby changing an electrical property of the CNT (Wang and Yeow, 2009). In the case of inert gases, charge transfer between the gas molecules and the CNT is negligible; rather, it is the resistance of the degassed CNT that changes with the adsorption of inert gases (Sumanasekera et al., 2000). The change in resistance is believed to be due to the change in the electron and hole free carrier lifetimes of the CNT (Sumanasekera et al., 2000).
CNT also have been used as a sensing element in a radiation sensor, where the CNT were used to form parallel plate electrodes (Ma et al., 2007). However, for any given bias voltage, the CNT-based sensor collected a smaller amount of charge than stainless steel electrodes. Contrary to the investigators' expectation, the bias voltage could not be lowered by substituting CNT electrodes for stainless steel electrodes.
CNT in the form of single-walled carbon nanotubes (SWCNT) have been used in a micromechanical system (MEMS) as a sensor of pressure (Stampfer et al., 2006; Helbling et al., 2009). The MEMS-based sensor was made of a double layer pressure sensor membrane containing layers of SiO2 and Al2O3, with the SWCNT embedded between the two layers. The SWCNT were in contact with source and drain metal electrodes, and additionally a gate electrode was used to bias the SWCNT in a transistor configuration. An applied differential pressure produced a strain in the pressure sensor membrane, which was transferred to the embedded SWCNT and resulted in a change in the resistance of the SWCNT (Yang and Han, 2000).
Despite the use of CNT in the areas of gas and pressure sensing, direct sensing of ions with high sensitivity using carbon nanomaterials and operating at low bias voltages has not been accomplished up to now.